With respect to a point-to-multipoint transmission on the PON (Passive Optical Network) wherein two or more local offices are connected via optical fibers to a central office, there has been proposed a scheme according to which: each local office is assigned pseudo-random spreading codes orthogonal to each other, and modulates an optical signal in accordance with the spreading codes assigned thereto and transmits the modulated optical signal; and the central office multiplexes such optical signals from the respective local offices and transmits over a long distance. A description will be given below of a conventional technique for optical frequency coding in an optical frequency region by use of the spreading codes.
FIG. 1 schematically shows the configuration of one channel and optical frequency coding (wavelength coding) in the optical code division multiplex communications system. At the transmitting side, a broadband optical signal 20 is emitted from a light source 10 for incidence to an encoder 11, which performs wavelength coding of the incident optical signal by permitting the passage therethrough of only wavelength components corresponding to selection wavelength components 31 of the encoder 11, providing an coded optical signal 21. The thus coded optical signal 21 is transmitted over an optical fiber 13 to a decoder 12 at the receiving side. The decoder 12 similarly permits the passage therethrough of only those codes of the optical signal 21 from the corresponding encoder 11 which are selected according to selection wavelength components 32 of the decoder, providing a decoded optical signal 22.
On the other hand, as shown in FIG. 1(c), when an optical signal is input to the decoder 12 from an encoder which does not correspond to the decoder and the input optical signal contains wavelength components 21′ based on the selection wavelength components 31′ of the encoder, all chips (optical frequencies or wavelengths) of the input optical signal are not allowed to pass through the decoder 12 according to its selection wavelength components 32; even if allowed to pass, some chips of the input optical signal are allowed, and hence the optical signal is not decoded into an appropriate optical signal but instead becomes an optical noise 22′. The encoder 11 and the decoder 12 mentioned herein are disclosed in non-patent document 1, for instance.
The wavelengths for use by the conventional encoder and decoder are specific for them, and the wavelength of the input optical signal 20 to the encoder 11 and the selection wavelength 31 of the encoder 11 are not allowed, in almost all cases, to deviate from their predetermined absolute wavelengths. This raises a problem that the receiving side is required to notify the transmitting side of the wavelength of the optical signal to be sent and the selection wavelength 31 of the encoder 11, whereas the transmitting side is required to calibrate the wavelength 20 of the light source 10 and the selection wavelength 31 of the encoder 11 in response to the notification.
A solution to this problem is proposed, for example, in non-patent document 2 and patent document 1 (issued Feb. 2, 1999). With the proposed method, light emitted from a broadband light source which has a wavelength width of several dozen nanometers, such as LED (Light Emitting Diode), is input to a Mach-Zehnder or Fabry-Perot filter made of a material with less temperature dependence of its selection wavelength, wherein the input light is subjected to wavelength coding by the selection of its wavelength through use of sine functions; that is, data sequences are each assigned a wavelength with a different period.
In conventional optical communications, a binary data sequence is transmitted using an intensity modulation scheme that represents the presence or absence of an optical signal, depending on whether the value of each piece of data sequence is a space or mark.
A proposal has also been made to apply a four-phase modulation technique now in use in radio communications to optical communications as well. This technique is to convert the optical phase of an optical signal of one wavelength into one of four predetermined phases in accordance with two data sequences.
For optical transmission of two or more data sequences in multiplexed form there are available an “Optical FDM (Optical Frequency Division Multiplex) or WDM (Wavelength Division Multiplex) method. In WDM-PON employing the optical wavelength division multiplex method, it is necessary to adjust the wavelengths of optical signals to be sent from respective local offices to ensure accurate signal multiplexing and demultiplexing. To avoid such wavelength adjustment, there has been proposed an optical communications system in which each local office modulates, according to data sequence, an optical signal received from the central office and sends back thereto the modulated optical signal (see, for instance, non-patent documents 3 and 4).
[Patent Document 1] Japanese Patent Application Kokai Publication No. H11-32029
[Non-Patent Document 1] Saeko Oshiba et al., “Experimental Study on Bit Rate Enhancement Using Time-Spread/Wavelength-Hop Optical Code Division Multiplexing,” 2002 Annual General Conference of the Institute of Electronics, Information and Communication Engineers of Japan, B-10-80
[Non-Patent Document 2] T. Pfeiffer et al., “High Speed Optical network for Asynchronous Multiuser Access Applying Periodic Spectral Coding of Broad Band Sources,” vol. 33, No. 25, pp. 2141-2142, 1997, Electronics Letters
[Non-Patent Document 3] Takeshi Imai et al., “The Inter-Operability of WDN-PON System ONU Using a Reflective SOA,” 2002 Society Conference of the Communications Society of the Institute of Electronics, Information and Communication Engineers of Japan, B-10-50
[Non-Patent Document 4] Satoshi Narukawa et al., “Transmission Characteristics of Wavelength Channel Data Rewriter Using Semiconductor Optical Amplifier,” 2003 Society Conference of Communication Society of the Institute of Electronics, Information and Communication Engineers of Japan, B-10-51